Free-standing Si Research Articles

Enhanced interface protection of freestanding Si anodes in carbon fiber nanotube by introducing MoO3 buffering layers

Developing Silicon (Si) based flexible and freestanding anodes is crucial for future wearable lithium ion batteries (LIBs) with satisfiable energy density. Improving the poor conductivity and unstable structure of Si in the lithiation/delithiation processes is a significantly important challenge. In this work, we synthesized flexible composites MoO3/Si@CFNT, in which MoO3 as a decorative layer and Si nanoparticles are well encapsulated into carbon fiber nanotube (CFNT). As freestanding lithium ion battery (LIB) anodes, the as-synthesized MoO3/Si@CFNT composites exhibit excellent Li+ storage performance for both half-cell and full-cell (610 mAh g−1 @200 cycles at 0.2 A g−1 for half-cell and 159 mAh g−1 @100 cycles at 0.5 A g−1, with a capacity retention rate of 85.9 % and almost 99 % coulombic efficiency for full-cell). We demonstrated that CFNT reserves sufficient space for the volume change of Si caused by lithiation, meanwhile, the enhanced interface protection of freestanding Si anodes in CFNT was achieved by introducing MoO3 buffering layers. Furthermore, we revealed the ion/electron transport kinetics of MoO3/Si@CFNT flexible electrode by GITT and ex-situ EIS measurements. This work offers a good example and inspiring insights into the design of many other Si based flexible anodes.

Conductive copper ion-reinforced sodium alginate aerogel based free-standing Si anode for Li-ion storage

Biomass materials with great advantages of wide accessibility, low cost and environmental benignity have favorable application prospects as Si anodes for lithium-ion batteries (LIBs). In order to alleviate the volume effect and improve the conductance of Si, we fabricated a conductive copper ion-reinforced sodium alginate (SA) aerogel based free-standing Si anode (Si@CNTs/Alg-Cu-II) containing silicon nanoparticles, sodium alginate aerogel, carbon nanotubes and Cu nanoparticles, via a simple, low-cost and sustainable process (under 1 × 10−1 Pa, 1 °C/min to 200 °C holding for 1 h). In the irregular lamellar porous aerogel, silicon particles with good dispersion are bound by macromolecular chains of SA via dynamic hydrogen bonds and subsequently restrained in the porous aerogel to release volume stress. The in situ generated Cu particles and well-dispersed carbon nanotubes (CNTs) construct the conductive network around Si particles to improve the electrical conductivity of the electrode. This unique structure delivers a capacity of 835.6 mAh/g after 100 cycles at 0.1 A/g based on the total mass of Si@CNTs/Alg-Cu-II composite (Si content of 25.62%). Moreover, it also shows outstanding stability under high current density (a capacity retention of 84.1% after 100 cycles at 0.5 A/g). The sustainable preparation method and excellent electrochemical performance of Si@CNTs/Alg-Cu-II provide a new strategy to develop high-performance free-standing Si-based anodes for LIBs.

Freestanding Serpentine Silicon Strips with Ultrahigh Stretchability over 300% for Wearable Electronics.

Well-functionalized electronic materials, such as silicon, in a stretchable format are desirable for high-performance wearable electronics. However, obtaining Si materials that meet the required stretchability of over 100% for wearable applications remains a significant challenge. Herein, a rational design strategy is proposed to achieve freestanding serpentine Si strips (FS-Si strips) with ultrahigh stretchability, fulfilling wearable requirements. The self-supporting feature makes the strips get rid of excessive constraints from substrates and enables them to deform with the minimum strain energy. Micrometer-scale thicknesses enhance robustness, and large diameter-to-width ratios effectively reduce strain concentration. Consequently, the FS-Si strips with the optimum design could withstand 300% stretch, bending, and torsion without fracturing, even under rough manual operation. They also exhibit excellent stability and durability over 50,000 cycles of 100% stretching cycles. For wearable applications, the FS-Si strips can maintain conformal contact with the skin and have a maximum stretchability of 120%. Moreover, they are electrically insensitive to large deformations, which ensure signal stability during their daily use. Combined with mature processing techniques and the excellent semiconductor properties of Si, FS-Si strips are promising core stretchable electronic materials for wearable electronics.

Self-Rolled-Up Ultrathin Single-Crystalline Silicon Nanomembranes for On-Chip Tubular Polarization Photodetectors.

Freestanding single-crystalline nanomembranes and their assembly have broad application potential in photodetectors for integrated chips. However, the release and self-assembly process of single-crystalline semiconductor nanomembranes still remains a great challenge in on-chip processing and functional integration, and photodetectors based on nanomembrane always suffer from limited absorption of nanoscale thickness. Here, a non-destructive releasing and rolling process is employed to prepare tubular photodetectors based on freestanding single-crystalline Si nanomembranes. Spontaneous release and self-assembly are achieved by residual strain introduced by lattice mismatch at the epitaxial interface of Si and Ge, and the intrinsic stress and strain distributions in self-rolled-up Si nanomembranes are analyzed experimentally and computationally. The advantages of light trapping and wide-angle optical coupling are realized by tubular geometry. This Si microtube device achieves reliable Ohmic contact and exhibits a photoresponsivity of over 330mA W-1 , a response time of 370 µs, and a light incident detection angle range of over 120°. Furthermore, the microtubular structure shows a distinct polarization angle-dependent light absorption, with a dichroic ratio of 1.24 achieved at 940nm. The proposed Si-based microtubes provide new possibilities for the construction of multifunctional chips for integrated circuit ecosystems in the More than Moore era.

In situ characterization of high-temperature mechanical behaviors of freestanding Si, BSAS and Yb2SiO5 environmental barrier coatings by three-point bending tests based on digital image correlation

The high-temperature mechanical properties and failure mechanisms of Si, BSAS and Yb2SiO5 ceramic materials are important for environmental barrier coatings. The Si coating, BSAS coating and Yb2SiO5 coating were prepared by the atmospheric plasma spray (APS) technique. The elastic modulus, fracture strength and fracture strain are determined by three-point bending combined with the digital image correlation (DIC) method. The results show that as the test temperature increases from 25 °C to 1000 °C, the elastic modulus of the Si coating and Yb2SiO5 coating decreases while the elastic modulus of the BSAS coating increases. And the fracture strength and fracture strain of the Si coating, BSAS coating and Yb2SiO5 coating increase. The elastic modulus is related to the interatomic distance and bonding strength of atoms. The fracture strength and fracture strain increase with the densification of the coating.

Fabrication of rutile TiO2 nanoarrays/free-standing diamond composite film and its field emission properties

Highly aligned TiO2 nanoarrays (TNAs) are fabricated on free-standing diamond (FSD) films and Si wafers (for comparison). Different anodizing times (e.g., 5, 15, 25, 45, and 60 min) are used to investigate the morphologies of TNAs and their effect on the electron field emission (EFE) properties. With the increase of the anodizing time, the features of TNAs grown on the FSD film undergo the transition from nanorods to nanopores and then to nanotubes. The XRD, Raman, and TEM results reveal that TNAs synthesized on the FSD film are preferentially oriented rutile phase, while those synthesized on the Si wafers show the anatase structure. Rutile TiO2 has a relatively low work function and is favorable to EFE performance. Importantly, the nanotubes on the FSD substrate with a 45 min anodizing time grow perpendicularly to each crystal face of the diamond grains and exhibit a superior EFE behavior, which can be turned on at a low field of 0.7 V/μm and attain a high current density of 0.6 mA/cm2 at an applied field of 1.6 V/μm. The enhanced EFE performance of TNAs/FSD composite film is ascribed to the synergetic effects among the electron acceleration layer played by the FSD film, more electron emission sites provided by the nanotubes with large aspect ratios, and lower work function of rutile TiO2. Moreover, the heterogeneity of the FSD substrate and TNAs also can decrease the interface barrier to make electrons transmit effectively.

Large-scale flexible membrane with resonant silicon nanowires for infrared visualization via efficient third harmonic generation

Nonlinear harmonic generation in nanostructures is one of the key topics in nanophotonics, as it allows infrared-to-visible light conversion at the nanoscale. Indeed, various nanophotonic designs have been developed to demonstrate high efficiencies for third harmonic generation (THG). However, fabrication approaches for these designs usually involve high-cost lithography-based technological steps. This work reports on the efficient THG in a free-standing Si nanowire array encapsulated into a flexible polymer membrane. High nonlinearity of Si material χ1111(3)≈2.62×10−19m2/V2 and light coupling with optical resonances in the nanowires yield a strong third-harmonic signal and efficient infrared (1200–2000 nm) to visible (400–666 nm) upconversion. The fabricated membranes demonstrate high flexibility and transparency, which make them convenient to use as infrared light visualizers.

Possibility of the existence of a topological defect in dynamic deformation of the free-standing ultrathin silicon wafer during MeV ion irradiation

We study the ion-irradiation-induced deformation of free-standing ultrathin Si wafers with a thickness of 8 μm. The time-response spectrum of the deformation was measured using a laser displacement meter with a time resolution of 1 ms. The results showed that the deformation appeared during irradiation and disappeared after irradiation. The deformation was composed of a fast deformation with a millisecond time constant and a slow deformation with a second time constant. We performed a model calculation to identify the deformation mechanisms. We found that the fast deformation originated from expansion or shrinkage of crystal lattice caused by beam heating and deduced that the slow deformation resulted from the topological defect formation in Si crystals. The relaxation time of the slow deformation is related to the coordination number of disappeared topological defects. In this experiment, we conclude that the deformation of Si crystals maintains reversible behavior in the formation of topological defects up to the coordination number 5.

Silicon nanowire/polymer membrane for infrared visualization via third-harmonic generation

Nonlinear harmonic generation in nanostructures is one of the key topics in nanophotonics, as it allows infrared-to-visible light conversion at the nanoscale.his work reports on the efficient third-harmonic generation in a free-standing Si nanowire array encapsulated into a polymer membrane. High nonlinearity of Si material X 31112.45 ≈ 10 –19m2 /V 2 and light coupling with optical resonances in the nanowires yield a strong third-harmonic signal and efficient infrared (1200 — 2000nm) to visible (400 — 666nm) upconversion. The fabricated membranes demonstrate high flexibility and semitransparency, which makes them convenient to use as visualizers.

(Invited) Enhanced Electron Phonon Scattering in Si Nanowires Covered By Oxide

In order to maintain gate controllability of electric current in nanoscale transistors, nanosheet or nanowire channels are required for future nodes. In Si covered by oxides, deformation potential greater than that in bulk has been used to reproduce electron mobility. Increase of the deformation potential results in decrease of phonon-limited mobility. However, physical origin of the increase of phonon scattering in nanostructures was not completely understood. Although atomistic calculations with H-terminated Si nanosheets partially explained the increase of phonon scattering, the amount of the increase of phonon scattering was not sufficient to reproduce experimental mobility or empirical values of deformation potential [1]. In this work, we calculated mobility in oxide covered semiconductor nanowires by Molecular dynamics (MD)-Landauer approach [2]. Based on MD simulations followed by transport calculations of tight-binding nonequilibrium Green’s function model, effects of the oxide layers on phonon scattering in the semiconductor nanowires was considered.The derived phonon-limited mobility of the free-standing Si nanowire (205 cm2V-1s-1) was almost the same as previously reported value in H-terminated Si nanowire [2], confirming the validity of our calculations. In the SiO2-covered Si nanowires, the mobility was decreased by half due to the SiO2 (95 cm2V-1s-1). Corresponding effective deformation potential was extracted by comparing the mobility calculated from MD-Landauer approach with the mobility calculated from Boltzmann transport equation. The corresponding deformation potential in SiO2-covered Si nanowires (D ac=22 eV) roughly agrees with the empirical deformation potentials in SOI films of the same thickness [3]. From the comparison between the free-standing and the SiO2-covered nanowires, we found the atomic displacements in the SiO2-covered Si nanowires were abnormally increased at the surface, which would result in the enhance of phonon scattering. Therefore, in order to precisely model the transport properties in future node’s transistors, atomistic descriptions of the interface between semiconductor and oxide were indispensable.[1] J. Li, JAP, 120, 174301, 2016.[2] T. Markussen, PRB, 95, 245210, 2017.[3] T. Ohashi, IEEE JEDS, 4, 278, 2016. Figure 1

Thin silicon via crack-assisted layer exfoliation for photoelectrochemical water splitting

SummarySilicon (Si) has been widely investigated as a feasible material for photoelectrochemical (PEC) water splitting. Compared to thick wafer-based Si, thin Si (<50 μm thickness) could concurrently minimize the material usage allowing the development of cost-effective and flexible photoelectrodes for integrable PEC cells. This work presents the design and fabrication of thin Si using crack-assisted layer exfoliation method through detailed optical simulations and a systematic investigation of the exfoliation method. Thin free-standing Si photoanodes with sub-50 μm thickness are demonstrated by incorporating a nickel oxide (NiOx) thin film as oxygen evolution catalyst, light-trapping surface structure, and a rear-pn+ junction, to generate a photo-current density of 23.43 mA/cm2 with an onset potential of 1.2 V (vs. RHE). Our work offers a general approach for the development of efficient and cost-effective photoelectrodes with Si films with important implications for flexible and wearable Si-based photovoltaics and (opto)electronic devices.

Solvent‐Induced Growth of Free‐Standing 2D Si Nanosheets

2D Si nanomaterials draw great interest owing to their fascinating properties and potential applications in electronic devices, catalysts, and energy storage and conversion devices. However, high-quality and large-scale synthesis of Si nanosheets remains a big challenge, despite the limited reports on their preparations via chemical exfoliation of layered Zintl silicide, magnesiothermic reduction of layered silicon oxide, and chemical vapor deposition. In this work, a facile, solution method to produce free-standing Si nanosheets in high yields and low cost, based on the reaction of commercial magnesium powder with trichlorosilane and tripropylamine in dichloromethane under mild conditions, is reported. The prepared Si nanosheets have an average thickness of ≈2nm and show photoluminescence. Experiments demonstrate that the key to the formation of Si nanosheets is the use of dichloromethane as a solvent. This method can be used to prepare Si nanosheets in large scale for various potential applications and possibly Si crystals with specific crystal morphology.

Large spin Hall effect in Si at room temperature

Silicon's weak intrinsic spin-orbit coupling and centrosymmetric crystal structure are a critical bottleneck to the development of Si spintronics, because they lead to an insignificant spin-Hall effect (spin current generation) and inverse spin-Hall effect (spin current detection). Here, we undertake current, magnetic field, crystallography dependent magnetoresistance and magneto thermal transport measurements to study the spin transport behavior in freestanding Si thin films. We observe a large spin-Hall magnetoresistance in both p-Si and n-Si at room temperature and it is an order of magnitude larger than that of Pt. One explanation of the unexpectedly large and efficient spin-Hall effect is spin-phonon coupling instead of spin-orbit coupling. The macroscopic origin of the spin-phonon coupling can be large strain gradients that can exist in the freestanding Si films. This discovery in a light, earth abundant and centrosymmetric material opens a new path of strain engineering to achieve spin dependent properties in technologically highly-developed materials.

High-Performance Freestanding Lithium-Ion Battery Si Anode by Weakening the Current-Collector Constraint

Lithium-ion batteries (LIB) with both high volumetric and gravimetric specific capacities are desirable for power sources in microsystems. Si is a promising anode material for the above LIBs. However, its huge volume change combined with strict constraint by the bulky current collector (and/or the substrate) during lithiation/delithiation causes a severe stress and thus rapid capacity fading. Strategies in the literatures help suppress the capacity fading but degrade the anode initial Coulombic efficiency and specific capacities. This work presents a freestanding Si anode with an ultrathin current collector. Unlike those in the literatures whose current collector (and/or the substrate) accounts for the main part, the Si film dominates in this freestanding anode. This causes weak constraint from the current collector, therefore, the Si film can expand/shrink easily and stress induced by Si volume change can be effectively released, thus resulting in good anode cyclability (98.6% retention over 150 cycles). Moreover, the dominant role of the Si film in this anode ensures the anode to have high volumetric (6989 mAh cm−3) and gravimetric (2107 mAh g−1) specific capacities even when both the Si and current collector are included for calculations. This freestanding anode also displays a high initial Coulombic efficiency of 92.8% due to its small surface area.

High-temperature, spectrally-selective, scalable, and flexible thin-film Si absorber and emitter

Solar thermal technologies have great potential to provide low-cost storage for solar energy. However, their efficiencies are limited by a lack of scalable, mechanically flexible, durable, yet highly-efficient spectrally-selective solar absorbers suitable for high temperatures at low solar concentrations. Here, we overcome these challenges by fabricating a scalable free-standing spectrally-selective thin-film Si absorber and emitter (SSTFS) composite. Its high-temperature emittance shows strong spectral selectivity, even at 595 °C. Thermal stability is proven by measuring optical properties before and after thermal cycling equivalent to one day of concentrated sunlight. Despite the use of crystalline Si, the fabricated SSTFS composite exhibits exceptional mechanical flexibility to cover most surface geometries. The SSTFS composite demonstrates the potential of high-temperature, efficient and flexible solar absorbers and thermal emitters to advance renewable solar energy with storage.

Surface Functionalization toward Biosensing via Free-Standing Si-OH Bonds on Nonoxidized Silicon Surfaces.

Usually hydroxyl groups present on top of oxidized silicon served as binding centers for a silanization reaction toward surface functionalization. In this study, we developed a novel surface functionalization strategy where hydroxyl functionalization on nonoxidized silicon surfaces are obtained. These surfaces were stable for several weeks even in ambient air at room temperature. This high stability indicates a strong spatial isolation of the hydroxyl groups because they keenly tend to undergo a condensation reaction, forming silicon oxide. To prove the applicability of the obtained hydroxylated silicon surface, we further modified the hydroxyl groups with a commonly used silane molecule, (3-aminopropyl)triethoxysilane (APTES). The functional amino groups of the APTES layer bonded to the surface were subsequently altered by N-maleoyl-β-alanin to generate a surface highly specific for the immobilization of thiol-containing biomolecules (like thiolated single-stranded DNA or cysteine-tagged proteins). All modification steps have been investigated by IR spectroscopic ellipsometry measurements.

Self-formed silver nanoparticles on freestanding silicon nanowire arrays featuring SERS performances.

Herein, the universal luminescence characteristics of porous Si nanowire arrays were exploited using a wide range of doping types and concentrations; we found that the dual-band photoluminescence intensities were correlated with the formation rates of Si nanowires with porous features; however, these intensities exhibited no evident dependence on the doping conditions. Furthermore, we demonstrated a facile and reliable transfer method implementing the freestanding Si nanowire arrays while maintaining the robust photoluminescence behaviors under bending conditions. The fabrication protocol, involving lateral etching locally at the nanowire ends, enabled the controlled formation of uniform and large-area transferred nanowires with vertical regularity. Without the additional deposition of Ag nanoparticles, these transferred Si nanowire films inherently possessed SERS sensing capability with a relative enhancement factor over 1.8 times that of the Si nanowires with electroless-deposited Ag nanoparticles, which could practically emerge as a functional design for the integration of practical biochip devices.

Molten-salt chemical exfoliation process for preparing two-dimensional mesoporous Si nanosheets as high-rate Li-storage anode

Two-dimensional (2D) materials have attracted enormous attention due to their functional applications in energy storage. In this work, a low-temperature molten-salt chemical exfoliation methodology is developed for producing free-standing 2D mesoporous Si through deintercalation of CaSi2 in excess molten AlCl3 at 195 °C. The average dimension of these sheets is 1.5 μm, and the thickness of a single sheet is approximately 10 nm. The as-prepared 2D Si has a Brunauer–Emmett–Teller surface area of 154 m2·g−1 and an average pore size of 5.87 nm. With this unique structure, the 2D Si exhibits superior Li-storage performance, including a reversible capacity of 2,974 mA·h·g−1 at 0.2 C, reversible capacities of 2,162, 1,947, and 1,527 mA·h·g−1 at 0.8, 2, and 5 C after 200 cycles, and a capacity retention of 357 mA·h·g−1 even at 30 C (90 A·g−1).

Carbon-silicon composite anode electrodes modified with MWCNT for high energy battery applications

In this study, we comparatively study the electrochemical characteristics of Si, Si-C and vacuum-assisted filtration fabrication of a novel free-standing Si@C/Mutli Wall Carbon Nanotubes (MWCNT) nanocomposite. The surfaces of the as-received Si nanaoparticles were coated with an amorphous carbon layer and homogenously anchored onto the surfaces of as-received MWCNTs by a simple vacuum filtration method. The samples were then analyzed with field emission scanning electron microscopy and X-ray diffraction (XRD) methods. Si@C/MWCNT samples have shown a stable capacity of 1290 mA h g−1 after 200 cycles. The results have proven that MWCNT’s large surface area, highly conductive network which can provide good contact between Si@C nanoparticles, tolerating large volume change sand suppressing aggregation of Si@C nanoparticles during charge/discharge processes. Such a comparison between the performances of carbon-MWCNT-metal materials is reasonably envisaged not only to be useful for understanding the individual contribution from MWCNT and metal but also to form a fundamental basis for energy storage applications. Free-standing Si-C/MWCNT nano paper has been successfully obtained by a facile vacuum filtration method.

Binder-free freestanding flexible Si nanoparticle–multi-walled carbon nanotube composite paper anodes for high energy Li-ion batteries

Abstract